The Relation of Cambial Zone Mechanical Strength to Growth and Irrigation of Almond [Prunus dulcis (Mill.) Webb.] Trees

Abstract

Commercial harvesting of almonds [Prunus dulcis (Mill.) Webb.] with mechanical shakers is economical, but may also cause severe damage to the cambial zone, leading to the establishment of a deadly cankerous disease, ceratocystis. Irrigation is often cut off for some period of time before harvest, anticipating a reduction in the damage as a result of an increased strength of adhesion within the cambial zone between the bark and the wood. Mechanical failure can occur in the cambial zone proper and in the differentiating cells on either side of the cambial zone. In this study the shear force per shear area in the cambial zone (cambial strength) was measured to represent mechanical failure due to shaker damage. Cambial strength of branches with intact bark tissues could be reversibly influenced by the level of tissue hydration, with high cambial strength associated with increased tissue hydration, presumably a result of an increase in the turgor of cambial zone cells. Methods of measuring cambial strength were developed for branches and trunks to avoid the effects of reversible turgor changes. Irrigation regimes imposed before harvest had a substantial and progressive influence on tree stem water potential, stomatal conductance, and growth rate of almond trees. Statistically significant levels of within seasonal differences in rate of growth, stomatal conductance and tree water deficits found under different irrigation regimes did not show any correlation with the within seasonal fluctuations in cambial strength. Cambial strength always showed a similar pattern and a similar magnitude of seasonal increase from spring to summer (as previous authors have reported), superimposed over the within seasonal fluctuations, despite significant differences in tree water stress. Therefore, this study suggests that irrigation cut off may be an ineffective practice for the purpose of increasing cambial strength.